In recent years, the speed and the capacity of signal transfer between central processing units (CPUs) of high-end servers or super-computers have been increased. Accordingly, in order to break through the limitation of electrical signal transfer, an optical interconnect using high speed optical transmission technology for a short range or middle range inter-CPU transmission has been attempted to be used.
An optical interconnect includes, for example, an optical transceiver that converts an electrical signal into an optical signal. For example, the optical interconnect transmits data in the form of an optical signal between a transmitter optical transmission device and a receiver optical transmission device via a transmission line, such as an array optical fiber. An example of an optical transmission unit for optical transmission is a vertical cavity surface emitting laser (VCSEL). VCSEL is a compact and low-power-consumption laser element capable of directly modulating an electrical current. In addition, an example of an optical receiving unit for optical transmission is a photodiode (PD) that receives an optical signal and converts the optical signal into an electrical signal. In order to support wide-band signal transmission between CPUs, high-speed optical transmission (e.g., 25 Gb/s) is employed.
In addition, in optical interconnects, a multimode fiber (MMF) that facilitates array-structured optical connection is employed as an optical transmission line, for example. In general, the diameter of the core of an MMF is 50 micrometers. In order to achieve optical connection with an MMF, there is a limit to reduce the detector diameter of a PD, which is an optical receiving device. As a result, it is difficult to reduce the parasitic capacitance and, thus, it is difficult to increase the bandwidth of the PD. Accordingly, in order to achieve a higher-speed optical receiving circuit, it is effective to use an equalizer that compensates for the bandwidth of the PD through equalization.
For example, an optical receiving circuit including a grounded-base amplifier circuit connected to a photodiode and a dummy circuit for generating a reference signal has been developed as an optical receiving circuit used for an optical receiver (refer to, for example, Japanese Laid-open Patent Publication No. 8-279717). In addition, as an equalizer connected to a transimpedance amplifier (TIA), an equalizing circuit having a transfer function that is the inverse number of a transfer function of the pole and zero of the upstream amplifier has been developed (refer to, for example, Japanese National Publication of International Patent Application No. 2011-525777).
Furthermore, an optical receiver including a circuit that reduces an amount of offset of a limit amplifier circuit in the final stage among a plurality of limit amplifier circuits has been developed (refer to, for example, Japanese Laid-open Patent Publication No. 2003-168933). Still furthermore, an optical receiver including a feedback TIA that immediately feeds back part of a signal output from a differential amplifier circuit to the input, a dummy PD that inputs a negative signal output from the differential amplifier circuit, and an equalizer connected downstream of the TIA has been developed (refer to, for example, Jin-Sung Youn et al., “10-Gb/s 850-nm CMOS OEIC Receiver with a Silicon Avalanche Photodetector”, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 2, FEBRUARY 2012, pp. 229-236).
In addition, as an optical receiving circuit for high-speed optical transmission, such as optical interconnect, a differential optical receiving circuit having a high resistance to crosstalk from a neighboring channel even in an array structure can be used, for example.
As an example of an input waveform and an output waveform of an existing differential optical front-end, an input waveform and an output waveform used in “10-Gb/s 850-nm CMOS OEIC Receiver with a Silicon Avalanche Photodetector” are described below with reference to FIG. 24. FIG. 24 illustrates, as a reference, an example of an output waveform of a differential optical front-end using a feedback TIA. For example, in the case of large signal input, an input signal is a 600-Opp signal. For example, when such a feedback TIA is used in a differential optical front-end, a dummy PD that receives a negative signal output from a differential amplifier circuit is connected to the differential optical front-end.
In FIG. 24, the abscissa represents the time (nanosec), and the ordinate represents the voltage (V). As illustrated in FIG. 24, each of a positive signal 2421 and a negative signal 2422 output from the differential optical front-end has a waveform shifted to one side and, thus, is non-symmetrical. The negative signal is a signal which has the positive signal reversed. Accordingly, the amplifier circuit has an output that has high linearity in the middle of the output range and its vicinity. In contrast, in the upper or lower limit region or its vicinity, the output is saturated. As a result, in particular, on a logic-1 side where the output is close to its upper or lower limit, an output signal tends to be non-linear.
To perform equalization using an equalizer, a signal is to be linearly amplified. However, according to the above-described existing technology, the input and output characteristics of a signal in a differential amplifier circuit is non-linear. Accordingly, it is difficult to improve the optical receiving characteristic using an equalizer, which is problematic.